A "solid" Check Engine Light (CEL/MIL) that remains illuminated immediately upon turning the ignition to the ON position—without the standard 2-second bulb check—indicates a critical hardware or low-level software failure. The ECU enters "limp mode," utilizing a backup processor, which typically results in rough engine operation, a restricted rev limit, or a no-start condition.

Overview#

During the power-on self-test, the ECU's main microcontroller (MCU) performs several low-level diagnostic checks. On OBD0 and OBD1 ECUs, these include:

• Checking Special Function Registers (SFRs) of the MCU.
• Verifying internal and external RAM integrity.
• Calculating and verifying the program Checksum (CS).
• Verifying Analog-to-Digital (A/D) converter operation.
• Verifying the 6260 mapping chip (if equipped).

If any of these checks fail, the MCU halts execution and triggers a solid CEL. On chipped (socketed) ECUs, the vast majority of solid CELs are caused by:

1. Bad ROM/Program: Corrupt ROM files, incorrect checksums, or faulty EPROM burning.
2. Improper Soldering: Cold solder joints, solder bridges (shorts), or open circuits on the socket, the 74HC373 latch, or the J1 jumper.

Diagnostic Procedure#

Follow these steps to isolate the cause of a solid CEL on a chipped OBD0 or OBD1 ECU.

Step 1: Test with a Stock ROM#

Rule out software issues before investigating hardware.

1. Burn an unmodified, stock program matching your specific ECU hardware (e.g., a stock P06 bin for a P06 ECU).
2. If the ECU is VTEC-converted, test with a stock non-VTEC program first to verify base functionality.
3. Install the chip and power the ECU.
4. Observe the CEL and fuel pump:
   • Normal Behavior: The CEL illuminates, the fuel pump primes for 2 seconds, and both then turn off.
   • Result: If the ECU functions normally with a stock program, your soldering is sound. The issue resides in the custom ROM (e.g., incorrect checksum or incompatible code base).

Step 2: Perform the J1 Jumper Test (OBD1 Only)#

If the solid CEL persists with a known good stock ROM, test the hardware bypass.

1. Locate the J1 jumper on the ECU board, which directs the ECU to read from the external EPROM instead of the internal ROM.
2. De-solder or cut one leg of the J1 jumper to disable the external chip socket.
3. Power up the ECU.

Important

If the CEL clears after cutting J1, the ECU's internal circuitry is functional. The fault is confirmed to be in the external socket, the 74HC373 latch, or the associated solder joints. If the CEL remains solid, the ECU hardware itself may be damaged.

Step 3: Inspect and Test Soldering#

If the J1 test isolates the issue to the socket area, perform a physical inspection:

1. Clean the PCB around the socket and latch with isopropyl alcohol to remove flux residue.
2. Inspect all solder joints under magnification for:
   • Cold joints: Dull, grey, or cracked solder.
   • Solder bridges: Unintended connections between adjacent pins.
   • Lack of penetration: Solder that failed to flow through the via to the top side of the board.
3. Use a digital multimeter in continuity mode to verify connections between the EPROM socket, latch, and MCU.

Step 4: Check Decoupling Capacitors#

If you have installed noise-filtering decoupling capacitors (e.g., C91 and C92 on JDM P30 ECUs), verify their specifications.

Caution

Using decoupling capacitors with incorrect values (e.g., 0.1µF instead of the recommended 10pF) can pull down critical signals like PSEN or ALE, triggering a solid CEL.

Related Resources#

• Introduction to ECU Chipping0
• ECU Troubleshooting0
• ::: widget error-codes :::